The aim of this proposal is to characterize the mechanisms by which halothane sensitizes the heart to the arrhythmogenic properties of exogenously administered catecholamines. The concurrent administration of epinephrine, whether applied subcutaneously for hemostasis or parenterally for hemodynamic effect, during halothane anesthesia is a frequent event which could adversely affect outcome of an otherwise routine operation. Awareness of the mechanisms involved in sensitization will permit pre-operative anticipation of patients particularly at risk thereby allowing rational pretreatment or modification of the anesthetic technique. With the advent of the new generation of adrenergic blocking agents, it is now possible to selectively antagonize the Alpha1 (prazosin) or Beta1 (metoprolol) receptor. Dogs will be anesthetized with halothane and the epinephrine dose required to provoke arrhythmias will be determined before and after administration of adrenergic blocking agents. In this manner, it should be possible to establish that the Beta- adrenergic receptor-effector mechanism mediates the sensitization. The separate components of this system will be examined to determine at which step halothane acts to increase Beta adrenergic responsiveness. Initially, the receptor number and binding affinity will be assessed by a radio-labeled ligand assay on canine myocardial biopsies obtained serially during anesthesia with halothane and correlated with circulating catecholamine levels. It is expected that the ambient neurotransmitter concentration will be lowered by the ganglionic blocking action of halothane resulting in "up regulation" of receptor concentration. Next, the Beta adrenergic receptor number will be maximally increased by drug pretreatment with 6-hydroxydopamine, and any additional sensitization by halothane when compared to enflurane will be due to post-receptor mechanisms. The coupling of the receptor to the effector, adenylate cyclase, is dependent to a large extent on factors influencing membrane fluidity. In an in vitro system with reticulocyte ghosts, the comparative effects of halothane and enflurane on the turnover methylated phospholipids will be examined as this is an important parameter of membrane fluidity. Another coupling factor, the GTP content, will also be assessed in this same in vitro system.